When you run smbd -V on your Snow Leopard installation, you'll see it's running SAMBA version 3.0.28a-apple. While I'm not sure how much difference the "-apple" makes, version 3.0.28a is old. Very old. In other words, it's riddled with bugs. Apple hasn't updated SAMBA in 3 years, and for Lion, they're dumping it altogether for something homegrown. The reason? SAMBA is now GPLv3.

Since Apple has licensed and implemented Microsoft Exchange in Snow Leopard and iOS, I'm guessing they'll license some SMB implementation from Microsoft as well for Lion. I take it the Exchange licensing deal was a first step and now Apple is ready to license more from Redmond.